The present invention relates to an optical disk access system having a focus servo mechanism control system to control the focus and the tracking position of a light generated by an optical head and illuminating an optical disk. More particularly, the invention relates to a focus mechanism control system which automatically adjusts the focussing of an optical beam in accordance with an operational mode (e.g., read, write, seek) of an optical disk system. This automatic focussing provides an optical disk system that can operate accurately and fast.
Optical disks can be used as large capacity external memories. This is because the distance between adjacent recording tracks on an optical disk is narrow, for example, several micrometers, in a radial direction of the optical disk. A large number of tracks can therefore be located on a single disk. Because of the large amount of data stored on an optical disk, an optical disk access system must operate at high speed and accurately access the tracks. Fast and accurate operation is necessary in order to respond to access requests from a computer system.
Optical disk systems generally have a seek mode, a write mode and a read mode. In the write mode, data is written to an optical disk in an optical disk system. Writing is accomplished by, for example, generating a strong light beam via a laser diode and focussing this beam on the tracks of the optical disk. The written data ca be read in the read mode. To read, a lower intensity beam is focused on the tracks, and the amount of the beam reflected indicates the data stored in the tracks. In the seek mode, a low intensity beam is focused on the optical disk just as in the read mode.
In each mode, the laser beam must exactly follow the track of the optical disk and must be focussed onto the individual bit locations within the track. To accomplish this operation, an optical disk system includes a focus servo mechanism and a track servo mechanism. The focus servo mechanism forms the servo operation relating to focussing the laser beam onto the individual bits of data on a disk. This focussing operation is controlled in accordance With a focus error signal which is based on the actual amount of light reflected from the optical disk.
The track servo mechanism performs the servo operation needed for the laser beam to exactly follow a track. The track servo mechanism uses a track error signal which is also based upon the actual amount of light reflected from the optical disk. In such servo mechanism, it is necessary to offset the focus position of the laser beam with respect to the surface of a disk in order to perform an optimum servo operation. As described in U.S. Pat. No. 4,707,648, the amount of focus offset is chosen at the point where the track error signal has a maximum amplitude level. By selecting the focus offset in this manner, this optical servo mechanism can control the tracking of the laser beam using large amplitude track error signals. The dynamic range of this large amplitude track error signal is large and therefore the servo mechanism is more stable. However, reading and writing will be performed with the same focus offset as used in the seek mode. As a result, writing errors occur because the focus is not optimum for either reading or writing. Furthermore, the write error is derived from the phenomenon that the reflectivity of the disk is changed as a result of the writ.RTM.operation, thus altering the amount of reflected light that improperly causes the servo mechanism to adapt to the change in the amount of reflected light.
Moreover, the offset level is determined in dependence upon the amplitude of the track error signal which is measured while the optical disk is rotating. If the track error signal which is generated during one rotation of the optical disk has a high frequency, the track error signal is more precisely measured. This is because many sampling points can be obtained to determine the offset level. However, a high frequency track error signal can only be obtained if an optical disk has a large off-center variation; that is, the disk center hole is not precisely centered with respect to the tracks on the disk. Since optical disks normally do not have large off centered conditions, it is difficult to obtain precise measurement of track error.